Cellulose fiber and derivatives are widely used in paper, absorbent products, food or food-related applications, pharmaceuticals, and in industrial applications. The main sources of cellulose fiber are wood pulp and cotton. The cellulose source and the cellulose processing conditions generally dictate the cellulose fiber characteristics, and therefore, the fiber's applicability for certain end uses. A need exists for cellulose fiber that is relatively inexpensive to process, yet is highly versatile, enabling its use in a variety of applications.
Kraft fiber, produced by a chemical kraft pulping method, provides an inexpensive source of cellulose fiber that generally provides final products with good brightness and strength characteristics. As such, it is widely used in paper applications. However, standard kraft fiber has limited applicability in downstream applications, such as cellulose derivative production, due to the chemical structure of the cellulose resulting from standard kraft pulping and bleaching. In general, standard kraft fiber contains too much residual hemicellulose and other naturally occurring materials that may interfere with the subsequent physical and/or chemical modification of the fiber. Moreover, standard kraft fiber has limited chemical functionality, and is generally rigid and not highly compressible.
In the standard kraft process a chemical reagent referred to as “white liquor” is combined with wood chips in a digester to carry out delignification. Delignification refers to the process whereby lignin bound to the cellulose fiber is removed due to its high solubility in hot alkaline solution. This process is often referred to as “cooking.” Typically, the white liquor is an alkaline aqueous solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S). Depending upon the wood species used and the desired end product, white liquor is added to the wood chips in sufficient quantity to provide a desired total alkali charge based on the dried weight of the wood.
Generally, the temperature of the wood/liquor mixture in the digester is maintained at about 145° C. to 170° C. for a total reaction time of about 1-3 hours. When digestion is complete the resulting kraft wood pulp is separated from the spent liquor (black liquor) which includes the used chemicals and dissolved lignin. Conventionally, the black liquor is burnt in a kraft recovery process to recover the sodium and sulphur chemicals for reuse.
At this stage, the kraft pulp exhibits a characteristic brownish color due to lignin residues that remain on the cellulose fiber. Following digestion and washing, the fiber is often bleached to remove additional lignin and whiten and brighten the fiber. Because bleaching chemicals are much more expensive than cooking chemicals, typically, as much lignin as possible is removed during the cooking process. However, it is understood that these processes need to be balanced because removing too much lignin can increase cellulose degradation. The typical Kappa number (the measure used to determine the amount of residual lignin in pulp) of softwood after cooking and prior to bleaching is in the range of 28 to 32.
Following digestion and washing, the fiber is generally bleached in multi-stage sequences, which traditionally comprise strongly acidic and strongly alkaline bleaching steps, including at least one alkaline step at or near the end of the bleaching sequence. Bleaching of wood pulp is generally conducted with the aim of selectively increasing the whiteness or brightness of the pulp, typically by removing lignin and other impurities, without negatively affecting physical properties. Bleaching of chemical pulps, such as kraft pulps, generally requires several different bleaching stages to achieve a desired brightness with good selectivity. Typically, a bleaching sequence employs stages conducted at alternating pH ranges. This alternation aids in the removal of impurities generated in the bleaching sequence, for example, by solubilizing the products of lignin breakdown. Thus, in general, it is expected that using a series of acidic stages in a bleaching sequence, such as three acidic stages in sequence, would not provide the same brightness as alternating acidic/alkaline stages, such as acidic-alkaline-acidic. For instance, a typical DEDED sequence produces a brighter product than a DEDAD sequence (where A refers to an acid treatment).
Traditionally, cellulose sources that were useful in the production of absorbent products or tissue were not also useful in the production of downstream cellulose derivatives, such as cellulose ethers and cellulose esters. The production of low viscosity cellulose derivatives from high viscosity cellulose raw materials, such as standard kraft fiber, requires additional manufacturing steps that would add significant cost while imparting unwanted by-products and reducing the overall quality of the cellulose derivative. Cotton linter and high alpha cellulose content sulfite pulps, which generally have a high degree of polymerization, are typically used in the manufacture of cellulose derivatives such as cellulose ethers and esters. However, production of cotton linters and sulfite fiber with a high degree of polymerization (DP) and/or viscosity is expensive due to 1) the cost of the starting material, in the case of cotton; 2) the high energy, chemical, and environmental costs of pulping and bleaching, in the case of sulfite pulps; and 3) the extensive purifying processes required, which applies in both cases. In addition to the high cost, there is a dwindling supply of sulfite pulps available to the market. Therefore, these fibers are very expensive, and have limited applicability in pulp and paper applications, for example, where higher purity or higher viscosity pulps may be required. For cellulose derivative manufacturers these pulps constitute a significant portion of their overall manufacturing cost. Thus, there exists a need for high purity, white, bright, low cost fibers, such as a kraft fiber, that may be used in the production of cellulose derivatives.
There is also a need for inexpensive cellulose materials that can be used in the manufacture of microcrystalline cellulose. Microcrystalline cellulose is widely used in food, pharmaceutical, cosmetic, and industrial applications, and is a purified crystalline form of partially depolymerized cellulose. The use of kraft fiber in microcrystalline cellulose production, without the addition of extensive post-bleaching processing steps, has heretofore been limited. Microcrystalline cellulose production generally requires a highly purified cellulosic starting material, which is acid hydrolyzed to remove amorphous segments of the cellulose chain. See U.S. Pat. No. 2,978,446 to Battista et al. and U.S. Pat. No. 5,346,589 to Braunstein et al. A low degree of polymerization of the chains upon removal of the amorphous segments of cellulose, termed the “level-off DP,” is frequently a starting point for microcrystalline cellulose production and its numerical value depends primarily on the source and the processing of the cellulose fibers. The dissolution of the non-crystalline segments from standard kraft fiber generally degrades the fiber to an extent that renders it unsuitable for most applications because of at least one of 1) remaining impurities; 2) a lack of sufficiently long crystalline segments; or 3) it results in a cellulose fiber having too high a degree of polymerization, typically in the range of 200 to 400, to make it useful in the production of microcrystalline cellulose. Kraft fiber having an increased alpha cellulose content, for example, would be desirable, as the kraft fiber may provide greater versatility in microcrystalline cellulose production and applications.
In the present disclosure, fiber having one or more of the described properties can be produced simply through modification of a kraft pulping plus bleaching process. Fiber of the present disclosure overcomes many of the limitations associated with known kraft fiber discussed herein.
The methods of the present disclosure result in products that have characteristics that are very surprising and contrary to those predicted based on the teachings of the prior art. Thus, the methods of the disclosure may provide products that are superior to the products of the prior art and can be more cost-effectively produced.